This invention relates to a process for recovery of vinyl acetate from a liquid mixture of vinyl acetate, paraffins and other impurities, and more particularly, relates to a two-step extraction process for the recovery of vinyl acetate from a liquid mixture of vinyl acetate, paraffins and other impurities.
Vinyl acetate may be manufactured by the oxidative addition of acetic acid to ethylene in the presence of a palladium catalyst. In general, either liquid phase or vapor phase processes are used commercially. In the liquid phase process, a mixture of ethylene and oxygen are fed into a single stage reactor which contains acetic acid, water and the catalyst. The products, i.e., vinyl acetate and acetaldehyde are separated from the exiting gas stream in a series of distillation columns. In the vapor phase process, a catalytic process is also used for vinyl acetate production, but is based on acetic acid addition to gaseous acetylene.
These vinyl acetate monomers are recovered and then used as feedstocks for various polymerization or copolymerization reactions to make polyvinyl acetate or other copolymers. The vinyl acetate monomer is usually stored in tanks before being used and has anti-oxidants added to it, in low percentages by weight, to prevent self-polymerization reactions in the storage tanks.
In more detail, one conventional copolymerization reaction involves copolymerizing ethylene with vinyl acetate in a reactor. The desired product is a polymer and the process is exothermic so that a diluent, usually iso-octane or some other paraffin, is used to control the amount of material entering the polymerization reaction and thereby control the heat generated in the polymerization reactor. The process also produces various byproducts such as oxygenates that may be alcohols, ketones, or aldehydes.
The liquid mixture from this reaction includes the anti-oxidants, byproduct contaminants, diluents, unreacted components (ethylene and vinyl acetate), as well as the polymer product. The polymer product is extracted by appropriate means and then any unreacted ethylene may be easily removed from the liquid mixture by a single distillation step. This leaves a liquid waste stream of vinyl acetate, diluents, oxygenates, and anti-oxidants. This waste stream is then diverted to waste storage tanks which may have previously contained almost anything, such as, for example, paraffins and/or olefins and heavy oils containing olefins and/or aromatics. When removed from these tanks, the waste stream will contain small amounts of such prior tank contents as additional contaminants or impurities.
Thus, such vinyl acetate polymerization or copolymerization processes may result in a liquid mixture of vinyl acetate and other impurities, such as paraffins, which form azeotropes with vinyl acetate and so may not be removed by conventional extractive distillation steps, as well as the other types of impurities noted before. As noted before, in addition to paraffins and/or olefins which may be present, such liquid mixtures may also include heavy oils which may be olefins and/or aromatics (with "heavy" meaning a boiling point higher than the boiling point of vinyl acetate). Other types of impurities may be water, anti-oxidants, and/or acetaldehyde (or other oxygenated compounds, such as methanol and acetone), or other alcohols, ketones, or aldehydes. It is conventional practice in the plastics industry to discard or burn such liquid waste stream mixtures of vinyl acetate and other impurities. However, in order to recover more vinyl acetate and to reduce the amount of waste material that must be reprocessed or discharged, a process for the recovery of vinyl acetate from such waste streams is desirable.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods are provided for the separation of vinyl acetate from liquid mixtures containing oxygenated compounds, paraffins, vinyl acetate, water, anti-oxidants, and other contaminants.